1. Field of the Invention
This invention relates to a voltage transformation device for a power supply circuit which is small in size and for which power transmission of a high efficiency is required and a driving method for the voltage transformation device.
2. Description of the Related Art
Conventionally, as a piezoelectric ceramic transformer which makes use of radial direction expansion vibrations of a disk, piezoelectric ceramic transformers disclosed in Japanese Patent Laid-Open Application No. 167504/92, Japanese Patent Laid-Open Application No. 338685/92 and Japanese Patent Laid-Open Application No. 67819/93 are known. Generally, those piezoelectric ceramic transformers positively make use of a radial direction expansion vibration of the third order mode of a disk as seen in FIGS. 1(a), 1(b) and 1(c) and operate in a high frequency band of 500 kHz or more. FIG. 1(a) is a plan view of a piezoelectric ceramic transformer, FIG. 1(b) is a sectional view taken along line A-Axe2x80x2 of FIG. 1(a), and FIG. 1(c) is a sectional view taken along line B-Bxe2x80x2 of FIG. 1(a). The piezoelectric ceramic transformer of the present conventional example includes high impedance part 72 having inner electrodes 74 opposing each other in a thickness direction at a central portion of a piezoelectric ceramic disk, insulating annular part 70 with no electrodes provided on the outer side of high impedance part 72 and low impedance part 71 having a plurality of pairs of inner electrodes 73 provided on the outer side of insulating annular part 70 and opposing each other in the thickness direction. One end of each of inner electrodes 74 of high impedance part 72 is exposed to a side end face of the piezoelectric ceramic disk without contacting with inner electrodes 73 of low impedance part 71, and every other ones of the ends of inner electrodes 74 are connected to each other by outer electrodes 79 and 79xe2x80x2, which are connected to electronic terminals 75 and 75xe2x80x2, respectively. Meanwhile, one end of each of inner electrodes 73 of low impedance part 71 is exposed to the side end face of the piezoelectric ceramic disk, and every other ones of the ends of inner electrodes 73 are connected to each other on the side end face by outer electrodes 78 and 78xe2x80x2. Further, outer electrodes 78 and 78xe2x80x2 are connected to electronic terminals 76 and 76xe2x80x2, respectively.
In the present example, high impedance part 72 is formed at the central portion of the disk while low impedance part 71 is formed at the peripheral portion of the disk, and in order to positively utilize a radial direction expansion vibration of the third order mode, inner electrodes 73 and 74 are formed such that they are individually centered at nodes of the vibrations of the radial direction expansion vibration of the third order mode.
In the present example, however, while the radial direction expansion vibration of the third order mode can be excited positively, it is impossible to intensely excite a radial direction expansion vibration basic mode, which is a vibration mode having a high effective electromechanical coupling coefficient and in which the disk is expanded and contracted uniformly. Further, where the piezoelectric ceramic transformer described above is applied to an AC adapter, since it is required that it operate in a frequency band lower than 150 kHz, there is a drawback that the piezoelectric ceramic transformer which operates in the radial direction expansion vibration of the third order mode inevitably makes the size of the transformer large.
It is an object of the present invention to provide a piezoelectric ceramic transformer of a small size which is high in energy conversion efficiency and operates in a low frequency band lower than 150 kHz.
A piezoelectric ceramic transformer according to the present invention is formed from a piezoelectric ceramic plate having an outer shape of a disk, and the piezoelectric plate is divided, where circular surfaces of the piezoelectric ceramic plate are defined as principal surfaces, in a thickness direction into two regions parallel to the principal surfaces and one of the two regions is formed as a driving part while the other region is formed as a generating part. Each of the driving part and the generating part forms a layering structure as a whole which comprises a plurality of inner electrode pairs formed in parallel to the principal surfaces and opposing each.other in the thickness direction and regions between the inner electrodes, and the regions between the inner electrodes are polarized so that any adjacent ones of the inner electrodes may have opposite polarization directions to each other. Further, an intermediate region between the driving part and the generating part is set as a non-polarized insulating layer. Furthermore, the inner electrodes which oppose each other are partially exposed to different side end faces of the piezoelectric ceramic plate so that the polarities thereof may be opposite to each other and are alternately connected to each other by outer electrodes and further connected to respective electronic terminals.
In a piezoelectric ceramic transformer of another form of the present invention, the piezoelectric ceramic plate is divided, where circular surfaces of the piezoelectric ceramic plate are defined as principal surfaces, in a thickness direction into three regions parallel to the principal surfaces and each having a symmetrical structure with respect to a center sectional plane and a central one of the three regions is formed as either a driving part or a generating part while the regions on the opposite sides of the central region are formed as either generating parts or driving parts different from that of the central region. Each of the driving part or parts and the generating parts or part forms a layering structure as a whole which comprises a plurality of inner electrode pairs formed in parallel to the main surfaces and opposing each other in the thickness direction and regions between the inner electrodes, and the regions between the inner electrodes are polarized such that any adjacent ones of the inner electrodes may have opposite polarization directions to each other. Further, intermediate regions between the driving part or parts and the generating parts or part are set as non-polarized insulating layers. Furthermore, the inner electrodes which oppose each other are partially exposed to different side end faces of the piezoelectric ceramic plate such that the polarities thereof may be opposite to each other and are alternately connected to each other by outer electrodes and further connected to respective electronic terminals.
Each of the piezoelectric ceramic transformers described above can be constructed in a similar manner using a square piezoelectric ceramic plate in place of the circular piezoelectric ceramic plate. Further, the outer electrodes fitted on the side end faces may be individually led out to locations in the proximity of the centers of the top and bottom principal surfaces of the piezoelectric porcelain plate, and electric leads may be led out from the locations. Here, in order to allow the piezoelectric ceramic transformer to operate in a radial direction (where a disk is used) or contour direction (where a square plate is used) expansion vibration basic mode, not the side end faces where antinodes of vibrations appear, but the locations in the proximity of the centers of the top and bottom principal surfaces where an influence of vibrations is comparatively little are selected as the locations from which the electric leads are to be led out.
Each of the piezoelectric ceramic transformers described above utilizes, when it is driven, a radial or contour direction expansion vibration basic mode which provides a high electromechanical coupling coefficient. This augments the power transmission efficiency of the piezoelectric ceramic transformer comparing with those of conventional piezoelectric ceramic transformers and allows further miniaturization of a piezoelectric ceramic transformer device.
The above and other objects, features, and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate preferred embodiments of the present invention by way of example.